Microvascular plasticity and neurovascular coupling in the pharmacotherapy of Parkinson's disease

Abstract: L-DOPA pharmacotherapy in Parkinson’s disease (PD) is associated with adverse effects occurring after a few years of treatment. Among these, dyskinesia (abnormal involuntary movements) is particularly common and potentially disabling. The causes behind dyskinesia are not completely understood, but intense research has identified a number of neuronal alterations associated with L-DOPA-induced dyskinesia in both PD patients and animal models of PD. In addition to neurons, the brain consists of a dense vascular network and supportive cells. This thesis has focused on non-neuronal aspects contributing to the pathophysiology of dyskinesia. In particular, it has addressed structural and functional changes affecting the microvasculature in the basal ganglia. Using rats with unilateral 6-hydroxydopamine (6-OHDA) lesions as an animal model of PD, the first paper of this thesis demonstrates that dyskinesia induced by either direct dopamine D1 agonism or L-DOPA, are indistinguishable at the level of angiogenic activity in the basal ganglia. Endothelial proliferation, nestin upregulation and downregulation of blood-brain barrier indices, were equally present in both D1- or L-DOPA-induced dyskinetic rats. Moreover, concomitant D1 stimulation by L-DOPA and a D2 antagonist, potentiated the angiogenic response without affecting the severity of dyskinesia. The second paper of this thesis shows that indices of angiogenesis and the expression of vascular endothelial growth factor (VEGF) are dose-dependently upregulated in the striatum and the substantia nigra pars reticulata following chronic L-DOPA treatment. Interestingly, VEGF immunoreactivity was mainly expressed in astrocytes in proximity to blood vessels. Induction of VEGF mRNA was seen in rat primary astrocytic cultures following incubation with D1 receptor stimulation. To verify the causal involvement of VEGF in the development of dyskinesia, 6-OHDA-lesioned rats were chronically co-treated with L-DOPA along with an inhibitor of VEGF receptor-signalling (vandetanib). Rats receiving vandetanib co-treatment, but not its vehicle, developed less severe dyskinesia and did not show a significant angiogenic response to L-DOPA. The occurrence of angiogenesis was investigated also on post mortem basal ganglia sections from PD patients. Histological indices of angiogenesis (i.e. the density of CD34- and nestin-immunopositive microvessels) were significantly higher in dyskinetic PD patients compared to non-dyskinetic PD cases and neurologically healthy controls. Striatal tissue samples from a separate set of dyskinetic patients showed a significant upregulation of VEGF mRNA. Prompted by the hypothesis that L-DOPA-induced angiogenesis may be accompanied by changes in regional cerebral blood flow (rCBF) in the affected regions, we carried out an extensive comparison of changes in rCBF and regional cerebral glucose utilization (rCGU) in unilaterally 6-OHDA-lesioned rats both at baseline and following the administration of L-DOPA (third article in the thesis). The results of this study demonstrate robust increases in rCBF at 60 minutes following the administration of L-DOPA (“ON L-DOPA”) in the same basal ganglia nuclei that show angiogenic activity upon chronic L-DOPA treatment. The rCBF response was not always paralleled by an increase in rCGU, pointing to a direct haemodynamic effect of the treatment. By comparing the passage of a tracer molecule across the blood-brain barrier in chronically L-DOPA-treated rats, we could finally show a significantly larger accumulation of the tracer in the striatum and the substantia nigra pars reticulata at 60 minutes (“ON”) compared to 24 hours (“OFF”) following the last peripheral L-DOPA injection. In conclusion, this thesis provides evidence that dopamine replacement therapy for PD does not only affect neurons, but that important plastic changes occur at the level of the microvasculature in the basal ganglia. These changes are tightly linked to the development of a dyskinetic motor response. On the basis of these findings, it is recommended that future therapeutic initiatives for PD also consider targets that are expressed in the microvasculature and/or neurovascular coupling mechanisms that are specific to the parkinsonian brain.